The present invention generally relates to circuits, and more particularly relates to a programmable integrated circuit having programmable capacitance.
A variety of circuits include programmable capacitance for tuning the circuits and/or for filtering. One example of a circuit typically including programmable capacitance is an amplifier that includes a feedback loop across one or more of the amplifier stages of the amplifier. FIG. 1 is a simplified schematic of a three stage amplifier 100 having a programmable capacitance circuit 105 that is configured to feedback an amplified signal across two of the amplification stages. Programmable capacitance circuit 105 may be used for a variety tuning purposes, such as reducing the bandwidth degradation from parasitic capacitance on the amplifier, for example, for high frequency amplification applications. The variable capacitor may also be used to compensate for parasitic circuit variation due to process variation between circuits. Further, the programmable capacitance circuit may be used to tune the feedback for a variable gain amplifier, wherein gain changes cause a set feedback capacitance to become unstable. Specifically, the feedback capacitor may be turned to stabilize this capacitive instability.
FIG. 2 is a further detailed schematic of programmable capacitance circuit 105, which includes a number of capacitance stages 110a-110n that are configured to be switched into the feedback loop. Note that reference nodes “A” in FIGS. 1 and 2 are matching nodes, and that reference nodes “B” in these figures are matching nodes. Each capacitance stage includes a capacitor 115 and a transistor 120. Each transistor, e.g., transistor 120a, is configured to switch its associated capacitance stage, e.g., 110a, and its associated capacitor, e.g., 115a, into the feedback loop to tune the capacitive feedback of the amplifier. One drawback of programmable capacitance circuit 105 is that transistors 120a-120n are typically relatively large so that the source-drain capacitance of these transistors (i.e., the capacitance of the transistors in series with the capacitors 115a-115n) approximately matches the capacitance of capacitors 115a-115n. 
Therefore, new circuits are needed that provide programmable capacitance and are configured to compensate for process variations, variable gain amplification, variable parasitic capacitance and the like.